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HLH (helix-loop-helix) proteins are a putative transcription factors family. Several HLH proteins take part in growth and development of an extensive range of tissues and species. NHLH1 functions as a DNA-binding protein and has a role in the regulation of cell-type determination in the developing nervous system.

Nescient Helix Loop Helix 2 (NHLH2) belongs to the basic helix-loop-helix (bHLH) transcription factor family. NHLH2 is expressed in developing and adult neuroendocrine tissues for example the pituitary and hypothalamus. NHLH2 can act as a DNA-binding protein and may be involved in the control of cell-type determination, probably in the developing nervous system.

HAND1 is a member of the basic helix-loop-helix family of transcription factors. HAND1 is one of two HAND proteins, two closely related family members that are irregularly expressed in the emerging ventricular chambers and have a key part in cardiac morphogenesis. Operating in a complementary manner, they work in the development of the right ventricle and aortic arch arteries, implicating them as mediators of congenital heart disease. Furthermore, HAND1 is obligatory for early trophoblast differentiation.

MAX protein is part of the basic helix-loop-helix leucine zipper (bHLHZ) family of transcription factors. MAX forms homodimers and heterodimers with Mad, Mxi1 and Myc. Myc is an oncoprotein implicated in cell proliferation, differentiation and apoptosis. The homodimers and heterodimers compete for a common DNA target site (the E box) and rearrangement among these dimer forms offers a complex system of transcriptional regulation. In contrast to Myc, which is exceedingly regulated throughout progression during the cell cycle, Max is very stable and is much more abundant than Myc.

MAX Dimerization Protein 3 (MXD3) belongs to the Myc superfamily of basic helix-loop-helix leucine zipper transcriptional regulators. MXD3 forms a heterodimer with the cofactor MAX that connects specific E-box DNA motifs in the promoters of target genes and regulates their transcription. Disturbance of the MAX-MXD3 complex is correlated with uncontrolled cell proliferation and tumorigenesis. Transcript variants of MXD3 encoding different isoforms have been described.

NEUROG3 is a common helix-loop-helix (bHLH) transcription factor which takes part in neurogenesis. NEUROG3 functions as a heterodimer with another bHLH protein. Mutations in this gene can result in congenital malabsorptive diarrhea 4 (DIAR4).

Inhibitor of DNA Binding 2 (ID2) is a part of the inhibitor of DNA binding family, whose members are transcriptional regulators that contain a helix-loop-helix (HLH) domain but not a basic domain. Members of the ID family inhibit the functions of basic helix-loop-helix transcription factors in a dominant-negative way by suppressing their heterodimerization partners through the HLH domains. ID2 play a role in negatively regulating cell differentiation and may be an inhibitor of tissue-specific gene expression.

Inhibitor of DNA Binding 1 (ID1) is a helix-loop-helix protein which can form heterodimers with members of the basic HLH family of transcription factors. ID1 is lacking DNA binding activity and thus can inhibit the DNA binding and transcriptional activation ability of basic HLH proteins with which it interacts. ID1 participates in cell growth, senescence, and differentiation.

HES7 belongs to the Hairy and enhancer of split family of bHLH transcription factors. The mouse ortholog of the HES7 is set by Notch signaling. The protein serves as a transcriptional repressor, and is involved in correct patterning of the axial skeleton. A mutation in HES7 has been shown to result in spondylocostal dysostosis. Multiple transcript variants encoding different isoforms have been discovered for HES7.

Hairy and enhancer of split 2 (HES2) belongs to the HES family and comprised of one basic helix-loop-helix (bHLH) domain and one orange domain. The HES family members form a complex with TLE, the mammalian homologue of Groucho, and this interaction is mediated by the carboxy terminal WRPW motif of the HES proteins. Activation of Notch signaling pathway initiates activation of HES family genes throughout the interaction between Notch intracellular domain and RBPSUH (CSL). HES2 is expressed in placenta, pancreatic cancer, colon cancer with RER, cervical cancer, and in head and neck tumors.

ATOH1 activates E box-dependent transcription in collaboration with TCF3/E47, but the activity is entirely provoked by the negative regulator of neurogenesis HES1. ATOH1 takes part in the differentiation of divisions of neural cells by activating E box-dependent transcription.

Collagen triple helix repeat-containing protein 1 (CTHRC1) functions as a negative regulator of collagen matrix deposition. CTHRC1 is a secreted 28kDa protein which is glycosylated and highly conserved from lower chordates to mammals. CTHRC1 is highly connected with calcified tissues and cartilaginous matrix, but not with endothelial cells. CTHRC1 is detected qualitatively in plasma of healthy human subjects. CTHRC1 plasma levels are also significantly elevated during pregnancy, in diabetes, in inflammatory and infectious conditions, in subjects with acute myeloid leukemia but not in subjects with solid cancers. The hormonal functions of CTHRC1 include regulation of lipid storage and cellular glycogen levels with potentially far-reaching implications for cell metabolism and physiology. CTHRC1 gene deletion leads to fatty liver (steatosis) formation in mice while others exhibited inactivation of the CTHRC1 gene also results in low bone mass.

HIF1A has a role as a master transcriptional monitor of the adaptive response to hypoxia. Under hypoxic conditions HIF1A activates the transcription of over 40 genes, including, erythropoietin, glucose transporters, glycolytic enzymes, vascular endothelial growth factor, and genes whose protein products increase oxygen release or facilitate metabolic adaptation to hypoxia. HIF1A functions as an essential role in embryonic vascularization, tumor angiogenesis and pathophysiology of ischemic disease.

Upstream Transcription Factor 1 (USF1) belongs to the basic helix-loop-helix leucine zipper family, and functions as a cellular transcription factor. USF1 activates transcription via pyrimidine-rich initiator (Inr) elements and E-box motifs. The USF1 gene has been linked to familial combined hyperlipidemia (FCHL). USF1 is a transcription factor which binds to a symmetrical DNA sequence (E-boxes) (5'-CACGTG-3') found in a various viral and cellular promoters.

Hypoxia-inducible factor-1 (HIF-1), identified as one of the transcription factors, has been found to play an essential role in cellular and systemic oxygen homeostasis. HIF-1 is a heterodimer composed of HIF-1b subunit and one of three subunits (Hif-1a, Hif-2 (or Hif-3)). The activation of Hif-1 (is closely associated with a variety of tumors and oncogenic pathways. Hif-1 (consists of DNA binding domain (DBD domain), Dimerization domain and C-terminal regulatory domains, including two transactivation domains (TAD), an oxygen-dependent degradation (ODD) domain, and inhibitory domains. Under hypoxic conditions HIF1A activates the transcription of more than 40 genes, including, erythropoietin, glucose transporters, glycolytic enzymes, VEGF, and other genes whose protein products increase oxygen delivery or facilitate metabolic adaptation to hypoxia. HIF-1A also plays a crucial role in embryonic vascularization, tumor angiogenesis and pathophysiology of ischemic disease. It binds to core DNA sequence 5'-[AG]CGTG-3' within the hypoxia response element (HRE) of target gene promoters. Activation involves recruitment of transcriptional coactivators such as CREBPB and EP300. Its activity is improved by interaction with both, NCOA1 or NCOA2. Interaction with redox regulatory protein APEX appears to activate CTAD and potentiates activation by NCOA1 and CREBBP. The induction is under reduced oxygen tension. HIF1A is also induced by a variety of receptor-mediated factors such as growth factors, cytokines, and circulatory factors for example PDGF, EGF, FGF-2, IGF-2, TGF-1 beta, HGF, TNF alpha, IL-1 beta, angiotensin-2 and thrombin. Nevertheless, this induction is less intense than that stimulated by hypoxia.

HIF1A has a role as a master transcriptional monitor of the adaptive response to hypoxia. Under hypoxic conditions HIF1A activates the transcription of over 40 genes, including, erythropoietin, glucose transporters, glycolytic enzymes, vascular endothelial growth factor, and genes whose protein products increase oxygen release or facilitate metabolic adaptation to hypoxia. HIF1A functions as an essential role in embryonic vascularization, tumor angiogenesis and pathophysiology of ischemic disease.